Vs. oxygen coverage

Figure 1. Effective Mo oxidation state vs. oxygen coverage.

Figure 3. Surface relaxation energy vs. oxygen coverage. The arrow indicates the relaxation energy measured for bulk M0O2.

Figure 6. Turnover frequency for isobutane formation (filled circles) and n-butane formation (open circles) vs. oxygen coverage.

The kinetics of the adsorption process and the structure of the oxide layer at potentials more positive than 1.5-1.7 V vs. RHE have been studied by a number of researchers. The total oxygen coverage generally... 

Fig. 17. O-induced surface stress on Si(lll) vs. O coverage. The slope of the solid line indicates an oxygen induced compressive surface stress of -5.7 N / m per monolayer O. The inset shows the resnlt for adsorption on Si(100). A slight tensile stress of order 0.2 N / m is indnced per monolayer O. Data from [91San], rescaled to account for one dimensional bending [97Ibal]...

In the absence of TCE and chlorine, the possible active species are holes (h+), anion vacancies, or anions (02 ), and hydroxyl radicals (OH ). At constant illumination and oxygen concentration, we may expect h+, and O2 concentrations to be approximately constant, and the dark adsorption to be a dominant variable. If kh+, or ko2- does not vary appreciably with the contaminant structure, the rate would depend clearly on the contaminant coverage as shown in Figme 2a, and the reaction would therefore occur via Langmuir-Hinshelwood mechanism. (Note only rates with conversions below 95% are correlated here (filled circles), as the 100% conversion data contains no kinetic information). This rate vs. d>r LH plot is smoother than those for koH or koH suggesting that non-OH species (holes, anion vacancies, or O2 ) are the active species reacting with an adsorbed contaminant. 

FIG. 28 Normalized steady-state diffusion-limited current vs. UME-interface separation for the reduction of oxygen at an UME approaching an air-water interface with 1-octadecanol monolayer coverage (O)- From top to bottom, the curves correspond to an uncompressed monolayer and surface pressures of 5, 10, 20, 30, 40, and 50 mN m . The solid lines represent the theoretical behavior for reversible transfer in an aerated atmosphere, with zero-order rate constants for oxygen transfer from air to water, h / Q mol cm s of 6.7, 3.7, 3.3, 2.5, 1.8, 1.7, and 1.3. (Reprinted from Ref. 19. Copyright 1998 American Chemical Society.)... 

Those experiments that did involve anodisation at more reasonable potentials, i.e. below oxygen evolution, suffered from an inability to characterise the initial PtOH species formed at coverages below a monolayer. Dickinson et ai (1975) systematically investigated the surface composition of a large number of Pt electrodes, polarised at various potentials in sulphuric acid, using XPS via the emersion approach. Figure 3.24 shows XPS spectra obtained from a Pt electrode after polarisation in sulphuric acid at 1.00 V and 1.5 V vs. SCE. 

More recent reviews on the electro catalysis of oxygen reduction in PEFC have been given in [23,49]. Surface chemistry of platinum, particularly the coverage of the metal surface with oxygen containing species above a potential of 750 mV vs. RHE caused difficulties in the kinetic description of the oxygen reduction reaction. 

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